As the sound source followed by image of cinema, etc., audio signals of plural channels are used. Such audio signal channels of plural channels are recorded on the assumption that they are respectively delivered to speakers disposed at both sides and the center of screen onto which image is projected and speakers disposed at the rear side or both lateral sides of listener, etc. so that they are reproduced. As stated above, by reproducing audio signals of plural channels by three-dimensionally disposed speakers, sound position followed by image and position of sound image actually heard can be in correspondence with each other. Thus, sound field having natural spreading of sound can be established.
Meanwhile, when listener attempts to appreciate (listen) sound from sound source consisting of audio signals of plural channels by using headphone device of the head portion mounting type, sound image by audio signals to both ears is localized within the head. As a result, position of the sound source and localized position of sound image are not in correspondence with each other. Thus, there results extremely unnatural sound image localization. Further, in reproduction using headphone device, localization positions of sound images of respective sound sources cannot be separately and independently reproduced. Even in the case where audio signals of left and right two channels are reproduced, the headphone device is adapted so that sound image is localized within the head unlike reproduction by speaker. As a result, sound is heard from one portion within the head. Thus, localization position of sound image cannot be separated, and it is possible to only generate extremely unnatural sound field.
In order to eliminate such problems and to obtain sound image equivalent to the case where reproduction is carried out by speaker even in the case where listener listens to audio signals of plural channels by using headphone device, there are used headphone devices of the stereo out-of-head sound image localization type adapted to measure or calculate transfer functions or impulse responses from speakers provided so as to respectively reproduce audio signals of respective channels in advance until both ears of listener to convolute them onto audio signals from sound source by digital filter, etc. thereafter to listen to it by headphone device to thereby position (localize) sound image outside the head.
As the stereo out-of-head sound image localization type headphone device of this type, there is proposed headphone device constituted as shown in FIG. 1. This headphone device 8 localizes reproduction sound image by audio signal to the left ear and audio signal to the right ear at the outside of the head.
The operation of the stereo out-of-head sound image localization type headphone device 8 will now be described.
First, prior to the operation of the stereo out-of-head sound image localization type headphone device 8, the case where audio signals are heard by two speakers installed or provided at the positions spaced from listener will be described. Audio signals are transmitted to left ear YL and right ear YR of listener M from sound source SL of the left side through paths having transfer functions of HLL, HLR, respectively. In addition, audio signals are transmitted to the left ear YL and the right year YR of listener from sound source SR of the right side through paths having transfer functions of HRL, HRR, respectively.
In order to reproduce the state where audio signals from left and right sound sources are reproduced by using two speakers by using headphone attached on the head portion shown in FIG. 1, audio signal Sal from sound source SL of the left side is caused to be through filter for realizing transfer function HLL so that audio signal Sbll for left ear is obtained and audio signal Sal is caused to be through filter for realizing transfer function HLR so that audio signal for right ear is obtained. In addition, audio signal Sar of sound source SR of the right side is caused to be through filter for realizing transfer function HRL so that audio signal Sbrl for left ear is obtained and audio signal Sar is caused to be through filter for realizing transfer function HRR so that audio signal Sbrr for right ear is obtained.
Then, left ear synthetic audio signal Sbl=(Sbll+Sbrl) and right ear synthetic signal Sbr=(Sblr+Sbrr) are obtained. Such an approach is employed to drive left and right headphone elements 6a, 6b of headphone 6 by these left and right synthetic audio signals Sbll, Sblr. Thus, listener can perceive or sense sound image as if sound source is disposed at sound sources SL and SR.
In this case, a more practical configuration of stereo out-of-head sound image localization type headphone device 8 conventionally proposed will now be described with reference to FIG. 1. This headphone device 8 comprises a first input terminal 1L supplied with audio signal Sal, a second input terminal 1R supplied with audio signal Sar, a second input terminal 1R supplied with audio signal Sar, A/D converters 2L, 2R for respectively converting respective audio signals Sal, Sar into corresponding digital signals, signal processing circuits 3L, 3R for implementing filtering (processing) to the respective audio signals Sal, Sar converted into digital signals, adders 7L, 7R for adding outputs of respective two systems, D/A converters 4L, 4R for converting added outputs of 2 systems into analog signals, and amplifiers 5L, 5R for amplifying analog audio signals outputted from the respective D/A converters 4L, 4R to supply them to left and right headphone elements 6a, 6b of the headphone 6.
In this example, one signal processing circuit 3L is constituted by two digital filters 10, 11 as shown in FIG. 3, wherein one digital filter 10 carries out convolution of impulse response of transfer function HLL with respect to audio signal Sal inputted through an input terminal 12 to form a left ear audio signal Sbll to output it from an output terminal 13, and the other digital filter 11 carries out convolution of impulse response of transfer function HLR with respect to audio signal Sal inputted through the input terminal 12 to form a right ear audio signal Sblr to output it from an output terminal 14.
The other signal processing circuit 3R is similarly constituted by two digital filters 10,11 as shown in FIG. 3, wherein one digital filter 10 carries out convolution of impulse response for realizing transfer function HRL with respect to an audio signal Sar inputted through the input terminal 12 to form a left ear audio signal Sbrl to output it from the output terminal 13, and the other digital filter 11 carries out convolution of impulse response for realizing transfer function HRR with respect to an audio signal Sar inputted through the input terminal 12 to form a right ear audio signal Sbrr to output it from the output terminal 14.
The above-mentioned impulse response has characteristic as shown in FIG. 4. To realize this, the respective digital filters 10, 11 are constituted by FIR type digital filter 15 as shown in FIG. 5, for example. This FIR type digital filter 15 comprises, as shown in FIG. 5, plural delay elements 16 cascade-connected having a predetermined delay quantity, plural coefficient multipliers 17 for multiplying inputted audio signal and audio signals delayed by respective delay elements 16 by coefficients for carrying out convolution of impulse response, and plural adders 18 for adding audio signals outputted from the respective coefficient multipliers 17.
For example, delay element 16 of the first stage of the digital filter 10 (11) in the signal processing circuit 3L delays, e.g., by one sampling period, audio signal Sal (or Sar) inputted through the input terminal 12, and delay element 16 of the i (i=2, 3, . . . )-th stage delays, similarly by one sampling period, delayed audio signal outputted from the preceding stage (i−1-th stage) to deliver it to delay element 16 of the succeeding stage (i+1-th stage). The coefficient multipliers 17 of respective stages respectively multiply input audio signal Sal and audio signals delayed in sequence at delay elements 16 of respective stages by coefficients for carrying out convolution of impulse response to deliver them to adders 18 of corresponding stages. The adders 18 of respective stages add, to outputs of adders 18 of preceding stages, outputs of coefficient multipliers 17 of corresponding stages to deliver them to adders 18 of succeeding stages. Namely, the adder 18 of the final stage carries out convolution of impulse response of transfer function HLL (HLR) with respect to audio signal Sal (Sar) inputted through the input terminal 12 to form left ear audio signal Sbll (right ear audio signal Sblr) to output it through the output terminal 13 (14).
Similarly, the adder 18 of the final stage of the digital filter 10 (11) in the signal processing circuit 3R carries out convolution of impulse response of transfer function HRL (HRR) with respect to audio signal Sal (Sar) inputted through the input terminal 12 to form left ear audio signal Sbrl (right ear audio signal Sbrr) to output it through the output terminal 13 (14).
In the case where the previously described two digital filters 10, 11 shown in FIG. 3 are realized by FIR type digital filter, they are collectively represented as shown in FIG. 6. The FIR type digital filter 20 shown in FIG. 6 is constituted as a single filter block by sharing cascade-connected plural delay elements 16 in obtaining outputs of two systems by using two FIR type digital filters 15 shown in FIG. 5. As stated above, as compared to the fact that two FIR type digital filters 15 as shown in FIG. 5 are prepared, FIR type digital filter 20 is constituted as shown in FIG. 6. Thus, the number of delay elements is reduced to one half so that the circuit scale becomes compact and signal processing operation quantity is reduced.
The above-described left ear audio signals Sbll, Sbrl outputted from signal processing circuits 3L, 3R of the headphone device 8 shown in FIG. 1 are added by one adder 7L so that left ear synthetic audio signal Sbl is obtained, and the above-described right ear audio signals Sblr, Sbrr outputted from signal processing circuits 3L, 3R are added by one adder 7L so that right ear synthetic audio signal Sbr is obtained. The left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr obtained in this way are respectively converted into analog signals at D/A converters 4L, 4R. The left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr converted into analog signals are respectively amplified by amplifiers 5L, 5R. The signals thus amplified are respectively delivered to left and right headphone elements ba, 6b of the headphone 6 so that they are reproduced. As the result of the fact that the left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr are reproduced in this way, listener M who has attached the headphone 6 perceives (senses) as if left and right two sound sources SL, SR are actually exist as shown in FIG. 2, thus making it possible to respectively localize reproduction sound images by the left ear synthetic audio signal Sbl and the right ear synthetic audio signal Sbr at the outside of the head.
On the other hand, in reproduction of audio signals using speaker, there are also instances where arrangement of speakers within the room is restricted. As a result, there are cases where it is difficult to dispose a large number of speakers within listening room. In view of the above, there has been proposed an example where a lesser number of speakers, eg., two speakers are used to constitute a large number of reproduction sound sources around listener as virtual sound source.
The example where these two speakers are used to constitute a large number of virtual speaker sound sources will be described with reference to FIGS. 7 and 8.
First, the principle of speaker unit 30 shown in FIG. 7 will be described with reference to FIG. 8.
In order to virtually reproduce sound source SO by using sound source SL and sound source SR, when it is assumed that transfer functions of audio signals from the sound source SL up to left ear YL and right ear TR of listener M are respectively HLL, HLR, transfer functions of audio signals from the sound source SR up to left ear YL, right ear YL of listener M are respectively HRL, HRR, and transfer functions of audio signals from sound source SO up to left ear YL, right ear YR of listener M are respectively HOL, HOR, the transfer (transmission) relationship between the sound source SL and the sound source SO is represented by the formula (1) described below and the transfer relationship between the sound source SR and the sound source SD is represented by the formula (2) described below.SL={(HOL×HRR−HOR×HRL)/(HLL×HRR−HLR×HRL)}×SO  (1)SR={(HOR×HLL−HOL×HLR)/(HLL×HRR−HLR×HRL)}×SO  (2)
Accordingly, such an approach is employed to allow audio signal Sao of sound source SO to be through filter which realizes transfer function portion of the formula (1) to thereby obtain left ear synthetic audio signal Sbl, and to allow the audio signal Sao to be through filter which realizes transfer function portion of the formula (2) to thereby obtain right ear synthetic audio signal Sbr to drive two speakers disposed at positions of sound sources SL, SR by these left ear and right ear synthetic audio signals Sbl, Sbr, thereby making it possible to localize virtual sound source as if audio signal Sao is generated from the position of sound source SO.
Speaker unit 30 for reproducing virtual sound source SO as described above can localize sound image of input signal inputted from two speakers to both ears at an ordinary position as shown in FIG. 7. This speaker unit 30 comprises an input terminal 21 supplied with audio signal Sao, an A/D converter 22 for converting the audio signal Sao into digital signal, and a signal processing section (unit) 23 for implementing filtering (processing) to the audio signal Sao converted into digital signal. The signal processing unit 23 is constituted by the previously described two digital filters 10, 11 as shown in FIG. 3, wherein one digital filter 10 convolutes impulse response corresponding to transfer function portion of the above-described formula (1) with respect to audio signal Sao to form left ear synthetic audio signal Sbl, and the other digital filter 11 convolutes impulse response corresponding to transfer function portion of the above-described formula (2) with respect to audio signal Sao to form right ear synthetic audio signal Sbr. With respect to digital filters 10, 11 for realizing transfer function, e.g., the previously described FIR type digital filter 15 shown in FIG. 5 or the previously described FIR type digital filter 20 shown in FIG. 6 is used, thereby making it possible to reduce the circuit scale.
The left ear and right ear synthetic audio signals Sbl, Sbr are respectively converted into analog signals by D/A converters 24L, 24R, and the left and right ear synthetic audio signals Sbl, Sbr of analog signal are respectively amplified by amplifiers 25L, 25R and are delivered to a left speaker 26L and a right speaker 26R. The left and right speakers 26L, 26R are respectively disposed at positions of sound sources SL, SR with respect to listener M.
As stated above, reproduction sound image by audio signal Sao can be localized at position of virtual sound source SO. Further, with respect to a larger number of sound sources, it is sufficient to carry out the above-described processing by the number of sound sources. Since a larger number of virtual speaker sound sources can be constituted from a lesser number of speaker sound sources by this method, the number of speakers can be reduced.
In the above-described headphone device and speaker unit, it is necessary for obtaining sufficient distance feeling with respect to virtual sound source to reproduce impulse response from respective sound sources to both ears obtained by measurement within reverberation room. Since such impulse response is vast digital quantity having long reverberation time, there is the problem that in the case where such a system is constituted by digital filter, its operation (computing) quantity and the scale become extremely large.
Further, in the above-described stereo out-of-head sound image localization type headphone device, in the case where positions of both ears of listener are changed during listening of virtual sound source, transfer function from electro-acoustic transducer (headphone element) which is reproduction sound source to both ears is not changed. Accordingly, sound is heard always from the same direction with respect to both ears irrespective of movement of both ears of listener. Thus, listener suffers from unnatural feeling such that directions in which sounds are heard are the same although he moves the head.
In addition, in the speaker unit, since transfer function from acoustic transducer (speaker) which is reproduction sound source to both ears is changed by change of positions of both ears of listener, primary position of virtual sound source is localized at unsuitable position. This always gives feeling of disagreement to listener.